Terahertz radiation (having a frequency from about 0.3 THz to about 30 THz) has many useful properties. For example, it can penetrate many materials, such as plastics, textile, paper, and wood, but is absorbed in metals. Terahertz radiation imaging and sensing can be useful in, for example, medical imaging, security and surveillance, materials characterization, and data communication. Security systems that use terahertz imaging can detect hidden objects under clothes. Terahertz radiation may interact with certain molecules to cause absorption of the radiation, providing spectroscopic fingerprints for the molecules. Terahertz radiation may be used to detect skin cancers not visible to the naked eye. Terahertz radiation can also produce useful information about properties, such as bound and unbound charge carrier modes, of solid state materials.